power: supply: bq256xx: Support to disable charger

To be able to control the charging process flexible, we need to able to
disable the charger. This commit will allow to disable the charger by
"echo 1 > /sys/class/power_supply/bq256xx-charger/charge_type"
(1 = POWER_SUPPLY_CHARGE_TYPE_NONE) and enable the charger by set it to
2/3 (POWER_SUPPLY_CHARGE_TYPE_TRICKLE/POWER_SUPPLY_CHARGE_TYPE_FAST)

Signed-off-by: Hermes Zhang <chenhuiz@axis.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>